Sodium vapor lamp having a grooved alumina arc tube with side rod heater retainer

ABSTRACT

The alumina arc tube of a high pressure sodium vapor arc discharge lamp has a spiral groove on the outer surface thereof. A refractory metal wire heater is wound around the arc tube and seated in the groove in order to maintain uniform spacing between the turns of the heater. A ceramic rod, abutting the arc tube, prevents displacement of the heater wire from the groove throughout lamp life.

United States Patent [19] Cohen et a1.

[ SODIUM VAPOR LAMP HAVING A GROOVED ALUMINA ARC TUBE WITH SIDE ROD HEATER RETAINER [75] Inventors: Sheppard Cohen, Danvers; Donald A. Richards0n,.Bever1y; Stephen F. Kimball, Georgetown, all of Mass.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

[22] Filed: Mar. 8, 1973 [21] Appl. No.: 339,327

[52] US. Cl 313/15, 313/180, 313/220, 313/229, 313/344 [51] Int. Cl. H0lj 7/24 [58] Field of Search 313/15, 25, 27, 180, 184, 313/220, 225, 228, 229, 344

[56] References Cited UNITED STATES PATENTS 2,765,416 10/1956 Beese et a1 313/15 X [451 Apr. 23, 1974 Ahsmann et a1 313/225 X 3,721,846 3/1973 Cohen 313/15 3,746,914 7/1973 Olson et al.... 313/225 X 3,757,158 9/1973 Kopelman 313/225 X 3,757,159 9/1973 Gutta et al 113/15 X Primary Examiner-Herman Karl Saalbach Assistant ExaminerSiegfried H. Grimm Attorney, Agent, or FirmJames Theodosopoulos 5 7] ABSTRACT 7 Claims, 2 Drawing Figures SODIUM VAPOR LAMP HAVING A GROOVED ALUMINA ARC TUBE WITH SIDE ROD HEATER RETAINER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to are discharge lamps and, in particular, to high pressure sodium vapor lamps.

2. Description of the Prior Art Within the past few years, high pressure sodium vapor lamps have become commercially useful, especially for outdoor lighting applications, because of their high efficiency, generally in excess of 100 lumens per watt. The sodium operating vapor pressure in such lamps can vary from several millimeters to about 1000 millimeters I-Ig.

Such lamps are called high pressure in order to distinguish them from low pressure sodium vapor lamps in which the sodium operating vapor pressure is in order of a few microns. Low pressure sodium lamps have been in use for about 30 or 40 years, but, although efficient, they produce an unattrative monochromatic yellow light. The color of light from high pressure sodium lamps is considerably improved over that from low pressure sodium lamps.

High pressure sodium lamps generally comprise an alumina ceramic arc tube and an arc tube fill of sodium, mercury and an inert gas. Examples of such lamps are shown in the following U.S. Pat. Nos. 3,248,590; 3,384,798; 3,448,319; 3,453,477; 3,485,343; 3,519,406; 3,521,108; 3,558,963 and 3,622,217.

One of the problems of high pressure sodium are discharge lamps relates to the starting thereof. Such lamps require a considerably higher starting voltage to initiate an arc discharge than do other types of are discharge lamps, such as fluorescent, mercury or metal halide. This higher starting voltage requirement necessitates the use of a special ballast for high pressure sodium lamps.

It is an object of this invention to provide a high pressure sodium vapor lamp having a reduced starting voltage, thereby eliminating the high voltage requirements of the ballast. Thus, a simpler more economical ballasting arrangement becomes quite practicable and, in fact, conventional mercury lamp ballasts can often be used with the lamps of this invention. This permits, in many cases, direct replacement of such mercury lamps by high pressure sodium lamps of this invention without any changes in the mercury lamp ballasts or fixtures.

SUMMARY OF THE INVENTION A high pressure sodium vapor lamp in accordance with this invention comprises an alumina arc tube having electrodes at its ends and containing a fill including sodium, mercury and an inert gas. A wire wound heater is disposed in heat transfer relationship with the arc tube in order to effect a substantial reduction in arc tube ignition voltage, as shown in co-pending application Ser. No. 214,000 filed Dec. 30, 1971, now U.S. Pat. No. 3,746,914 assigned to the instant assignee, the disclosure of which is incorporated herein by reference.

In a co-pending application, Ser. No. 339,326, entitled Sodium Vapor Lamp Having An Improved Grooved Alumina Arc Tube, filed on even date herewith, identified as Docket No. 7326L, same assignee, the wire wound heater is seated in grooves formed by ,raised shoulders spirally encircling the arc tube in order BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view, partly in section, of a lamp in accordance with this invention.

FIG. 2 is an expanded sectional view of the arc tube of the lamp of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT A high pressure sodium vapor lamp in accordance with this invention comprises an outer glass envelope 1 which can be of elongated ovoid shape, such as is commonly used in high pressure sodium lamps, or of bulbous shape, such as is commonly used in mercury vapor and metal halide lamps. The neck of the envelope is closed by a reentrant stem 2 having a press 3 through which extend stiff lead-in wires 4,5 connected at their outer ends to a threaded shell 6 and center contact 7 of a conventional screw base.

Disposed within envelope 1 is an alumina arc tube 8, having raised shoulders 30 forming grooves 21, sealed at its lower end by niobium end cap 9 and at its upper end by niobium end cap 10. Niobium tubes 11, 12 are brazed or welded to end caps 9,10 and are used to support are tube 8 as well as to conduct current to electrodes 13,14 within the arc tube. In addition, one of the niobium tubes serves as an exhaust tube during manufacture and is used to introduce a fill including an inert gas (e.g. argon or xenon), sodium and mercury into the arc tube, after which the niobium tube is sealed, such as by a cold weld.

Arc tube 8 is supported within envelope 1 by a structure consisting of side support wire 15, vertical support wire 16, metal strap 17 and horizontal metal strap 18. Support wire 15 is welded to lead-in wire 5, support wire 16 is welded to wire 15, strap 17 is welded to wire 16, strap 18 is welded to strap 17 and strap 18 is connected to upper niobium tube 12. Electrical connection between lead-in wire 5 and upper electrode 14 is established by said structure.

The lower end of arc tube 8 is supported by metal strap 19 which is connected between niobium tube 11 and support wire 20, with support wire 20 being welded to lead-in wire 4. Electrical connection is also established thereby between lead-in wire 4 and lower electrode 13.

Encircling arc tube 8 and seated in groove 21 is a wire wound heater 22 of the type disclosed in copending application Ser. No. 214,000. The upper and lower ends of heater 22 are electrically connected, respectively, to the lower and upper electrodes in order to further reduce the lamp starting voltage, as disclosed in co-pending application Ser. No. 266,294, now U.S. Pat. No. 3,721,846.

The upper end of a heater 22 is connected to a wire 23 which is embedded in glass rod 24. Glass rod 24 is supported from side wire by two wires 25 which are welded to side wire I5 and embedded in glass rod 24. Also embedded in glass rod 24 is another wire 26 one end of which is connected to support wire by a thin wire 27. A U shaped bimetallic switch 28 makes electrical connection between wire 23 and wire 26. The result of this electrical arrangement is that the voltage applied to lower electrode 13 is also applied to the upper end of heater 22. Switch 28 has a predetermined opening which exceeds the temperature at which are tube ignition occurs. Upon opening switch 28, heater 22 is electrically removed from the circuit.

The lower end of heater 22 is electrically connected to side support wire 15 by a similar arrangement of wires 23, 25' and 26', glass rod 24' and switch 28. Switch 28 also opens after arc tube ignition to electrically isolate heater 22.

Attached to the upper and lower ends of side wire 15 are spring fingers 31 which press against the interior wall of envelope 1 and provide increased support for v the arc tube. Also disposed on the lower end of side wire 15 are ring getters 29, since envelope 1 contains a vacuum to reduce arc tube heat losses.

Shoulders which form grooves 21 of arc tube 8 are raised, instead of machining a groove into the arc tube wall, in order to avoid thinning the arc tube wall at the grooves. The manufacture of such an improved grooved arc tube is described in co-pending application Ser. No. 339,326, Docket No. 7326L.

However, in the instant invention, there is an insulative rod 32 abutting arc tube 8 in order to prevent heater 22 from being displaced from groove 21. It is possible, without rod 32, for such displacement to occur if the lamp is subjected to vibration during operation. It is also possible for such displacement to occur if the lamp is operated in a vertical position, especially when heater 22 is a coiled coil and can sag out of the groove during the life of the lamp.

In order to permit heater 22 to be operated directly off the line voltage at which the lamp is operated, the

length of heater wire necessary for satisfactory operation of heater 22 is sufficiently great that heater 22 preferably consists of a coiled coil. That is to say; the heater wire is primary coiled on a mandrel and then, after mandrel removahthe primary coiled wire is secondary coiled, that is, wound around arc tube 8 in grooves 21. This minimizes the number of secondary turns on arc tube 8. To illustrate, in one example, heater 22 consisted of 4880 mm of 2% mil tungsten wire. If this wire were wound directly on arc tube 8, it wound require winding at the rate of about 56 turns per inch (TPI). I-Iowever, when the wire is first primary coiled on a 12 mil mandrel, it can then be wound on arc tube 8 at the rate of about 4 TPI. A winding rate of 4 TPI is considerably more preferable than 56 TPI for several reasons. There will be less interception of light from the arc tube by the heater wire itself. And there will be fewer numbers of raised shoulders 30 on arc tube 8; an excessive number of such shoulders can reduce the light transmission of the arc tube wall by causing a significant increase in the effective wall thickness of the arc tube. I

But the desirability of using a coiled coil for heater 22 results in disadvantages which this invention overcomes. A primary coiled wire cannot be wound on are tube 8 as tightly as a straight wire can. And a primary coiled wire is more likely to sag during vertical lamp operation than a straight wire. Rod 32, abutting arc tube 8, solves these problems by retaining the heater wire, that is, preventing the heater wire from sagging out of grooves 21. Rod 32 should be an insulative rod to prevent shorting out the turns of heater 22 and should be heat resistant, since the arc tube operating temperature is about 1200C. The diameter of rod 32 should be small, say, less than 40percent of the diameter of arc tube 8, in order to avoid interception of a significant amount of light from arc tube 8.

In the embodiment shown in the drawings, rod 32 was an alumina ceramic rod slightly longer than arc tube 8. Rod 32 was drawn against arc tube 8 by tie wires 33 which were wrapped around both rod 32 and are tube 8 and securely tied, such as by twisting. In addition, rod 32 was supported by bent rods 34, inserted into holes in the ends of rod 32. The other ends of rods 34 were fastened, such as by welding, to side support wire 15.

In this example, rod 30 had a diameter of mils and arc tube 8 had a diameter of 345 mils. The lengths of rod 30 and are tube 8 were five inches and 4 36 inches respectively.

Lamps made in accordance with this invention were o aeratd sv womiaurs'1h a vea'iaar acsifiomfiar severe conditions of continuous shock with no significant movement of heater 22. Without side rod 32 to retain the heater, lamps operated under the same conditions had excessive turn displacement in less than 50 hours.

We claim:

1. A high pressure sodium vapor lamp comprising: a glass envelope; an alumina arc tube disposed within said envelope and having electrodes sealed therein at the ends thereof and containing a filling including sodium, mercury and an inert gas, said are tube having a spiral groove formed by raised shoulders on its outer surface extending from about one electrode to the other, the shoulders defining said groove being raised above the outer surface of said are tube; a heater of refractory metal wire wrapped around said arc tube and seated in said groove; and an insulative heater-retaining side rod parallel to, and abutting, said arc tube.

2. The lamp of claim 1 wherein said groove is a channel between two parallel raised shoulders on said outer surface.

3. The lamp of claim 1 comprising, in addition, a thermal switch, one end of said heater being electrically connected to the electrode at the opposite end of said arc tube through said thermal switch, said switch being normally closed during non-operation of said lamp and opening after ignition of the arc tube and remaining open during normal lamp operation.

4. The lamp of claim 1 wherein said heater is a coiled coil.

5. The lamp of claim 1 wherein said side rod is less than about 40 percent of the diameter of said are tube.

6. The lamp of claim 1 wherein said side rod is made of alumina.

7. The lamp of claim 1 wherein said side rod is tied to said are tube with wire ties. 

1. A high pressure sodium vapor lamp comprising: a glass envelope; an alumina arc tube disposed within said envelope and having electrodes sealed therein at the ends thereof and containing a filling including sodium, mercury and an inert gas, said arc tube having a spiral groove formed by raised shoulders on its outer surface extending from about one electrode to the other, the shoulders defining said groove being raised above the outer surface of said arc tube; a heater of refractory metal wire wrapped around said arc tube and seated in said groove; and an insulative heater-retaining side rod parallel to, and abutting, said arc tube.
 2. The lamp of claim 1 wherein said groove is a channel between two parallel raised shoulders on said outer surface.
 3. The lamp of claim 1 comprising, in addition, a thermal switch, one end of said heater being electrically connected to the electrode at the opposite end of said arc tube through said thermal switch, said switch being normally closed during non-operation of said lamp and opening after ignition of the arc tube and remaining open during normal lamp operation.
 4. The lamp of claim 1 wherein said heater is a coiled coil.
 5. The lamp of claim 1 wherein said side rod is less than about 40 percent of the diameter of said arc tube.
 6. The lamp of claim 1 wherein said side rod is made of alumina.
 7. The lamp of claim 1 wherein said side rod is tied to said arc tube with wire ties. 